632
Susan S. Bell, Mark S. Fonseca, and Nathaniel B. Stafford
T. testudinum patches and used this to evaluate revegetation of disturbed areas. With the creation and use
of hydrodynamic or spatial models on the rise, physical forces appear to be of tremendous importance
to seagrass patch dynamics, and descriptive and predictive models will be important null models in future studies of the structure and function of seagrass
landscapes.
4. Gap Dynamics
In the terrestrial realm, the importance of gap dynamics is well known for forest and grassland systems, with implications for landscape structure. The
application of gap analysis to seagrass landscapes
(e.g. Bell et al., 1999) further expanded the use of
landscape methodologies to submerged habitats (see
also Duarte et al., Chapter 11). If gap formation is
intense and persistent, then a spatial mosaic characterized by areas with missing elements can result.
Moreover, gap formation may change the amount
of edge habitat or connectivity of the landscape.
In seagrass systems such gaps may be produced
by physical disturbance causing localized blowouts
(e.g. Patriquin, 1975) or by bioturbating organisms
(e.g. rays or horseshoe crabs), whose sediment processing frequently leaves pits within seagrass areas,
especially species with shallow, fragile root/rhizome
architecture. Anthropogenic effects such as boat propeller scars may also cause the removal of seagrass
within a bed (e.g. Bell et al., 2002). Macroalgal accumulations (mats) within seagrass beds may also
produce gaps within a seagrass bed if the mats are
substantial in size and persistent (Holmquist, 1992;
Nelson and Lee, 2001). In a seagrass system studied
by Bell et al. (1997), such gaps were visibly obvious, variable in size, and generally short-lived (<1.5
year) in the presence of the pioneer seagrass species
H. wrightii. Gap area has also been linked to extreme sedimentation events (Bell et al., 1999), again
suggesting that hydrodynamics and storms may play
a major role in modifying seagrass landscapes, and
that gap dynamics may be an expression of landscape
scale physical energy regimes.
B. Landscape Studies of Seagrass Habitat
1. Faunal Interactions
Having established that seagrass plants can be studied at the landscape scale, an additional area of inquiry is whether landscape principles applied to seagrasses provide new insights into plant–animal habitat relationships (see Chapters 20–22, for additional
discussions of animal–seagrass relationships). More
specifically, how does a landscape perspective apply
to the multitude of organisms that seagrass habitats support? Many worthwhile studies (e.g. Attrill
et al., 2000; Lee et al., 2001) have discussed the
importance of the physical structure of seagrass to
fauna, but since these do not explicitly relate to landscape principles they are not included here. There
have been numerous studies, however, that have examined how the spatial arrangement of the seagrass
landscape affects the fauna living within, on, and
above the seagrass vegetation.
Animal behavior within landscapes can be related to concepts of connectivity or the degree to
which landscape arrangement facilitates movement
between patches. When connectivity is high, patch
isolation is low and animals can percolate across
the habitat. However, at some point, declining connectivity may result in an inability of organisms to
percolate across a landscape (With and Crist, 1995).
Exactly how an organism perceives suitable habitat (e.g. seagrass) is a critical aspect of all these
discussions.
In a test of the effects of plant arrangement on the
recruitment of copepods in New Zealand, Bell and
Hicks (1991) placed artificial seagrass units (ASUs)
inside, on the edge of, and outside seagrass beds.
Significant differences in copepod recruitment to
the ASUs were found as a result of border type,
and the importance of studying landscape features
of the seagrass in conjunction with fauna was emphasized (Bell and Hicks, 1991). Kurdziel and Bell
(1992) also used ASUs to demonstrate that meiofaunal copepods could utilize small patches as corridors
for dispersal within a landscape. In both these studies
the utilization of ASUs, which allow experimenters
to control blade density, blade length, and position
of patches, has been quite useful for examining responses of small organisms to landscape features.
Where patches of seagrass are large and continuous,
however, construction of ASUs becomes logistically
problematic, thereby rendering them relatively less
attractive experimentally.
2. Patchiness and Macrofauna
Studies of macrofauna and seagrass landscapes have
provided some of the best information on landscape
Susan S. Bell, Mark S. Fonseca, and Nathaniel B. Stafford
T. testudinum patches and used this to evaluate revegetation of disturbed areas. With the creation and use
of hydrodynamic or spatial models on the rise, physical forces appear to be of tremendous importance
to seagrass patch dynamics, and descriptive and predictive models will be important null models in future studies of the structure and function of seagrass
landscapes.
4. Gap Dynamics
In the terrestrial realm, the importance of gap dynamics is well known for forest and grassland systems, with implications for landscape structure. The
application of gap analysis to seagrass landscapes
(e.g. Bell et al., 1999) further expanded the use of
landscape methodologies to submerged habitats (see
also Duarte et al., Chapter 11). If gap formation is
intense and persistent, then a spatial mosaic characterized by areas with missing elements can result.
Moreover, gap formation may change the amount
of edge habitat or connectivity of the landscape.
In seagrass systems such gaps may be produced
by physical disturbance causing localized blowouts
(e.g. Patriquin, 1975) or by bioturbating organisms
(e.g. rays or horseshoe crabs), whose sediment processing frequently leaves pits within seagrass areas,
especially species with shallow, fragile root/rhizome
architecture. Anthropogenic effects such as boat propeller scars may also cause the removal of seagrass
within a bed (e.g. Bell et al., 2002). Macroalgal accumulations (mats) within seagrass beds may also
produce gaps within a seagrass bed if the mats are
substantial in size and persistent (Holmquist, 1992;
Nelson and Lee, 2001). In a seagrass system studied
by Bell et al. (1997), such gaps were visibly obvious, variable in size, and generally short-lived (<1.5
year) in the presence of the pioneer seagrass species
H. wrightii. Gap area has also been linked to extreme sedimentation events (Bell et al., 1999), again
suggesting that hydrodynamics and storms may play
a major role in modifying seagrass landscapes, and
that gap dynamics may be an expression of landscape
scale physical energy regimes.
B. Landscape Studies of Seagrass Habitat
1. Faunal Interactions
Having established that seagrass plants can be studied at the landscape scale, an additional area of inquiry is whether landscape principles applied to seagrasses provide new insights into plant–animal habitat relationships (see Chapters 20–22, for additional
discussions of animal–seagrass relationships). More
specifically, how does a landscape perspective apply
to the multitude of organisms that seagrass habitats support? Many worthwhile studies (e.g. Attrill
et al., 2000; Lee et al., 2001) have discussed the
importance of the physical structure of seagrass to
fauna, but since these do not explicitly relate to landscape principles they are not included here. There
have been numerous studies, however, that have examined how the spatial arrangement of the seagrass
landscape affects the fauna living within, on, and
above the seagrass vegetation.
Animal behavior within landscapes can be related to concepts of connectivity or the degree to
which landscape arrangement facilitates movement
between patches. When connectivity is high, patch
isolation is low and animals can percolate across
the habitat. However, at some point, declining connectivity may result in an inability of organisms to
percolate across a landscape (With and Crist, 1995).
Exactly how an organism perceives suitable habitat (e.g. seagrass) is a critical aspect of all these
discussions.
In a test of the effects of plant arrangement on the
recruitment of copepods in New Zealand, Bell and
Hicks (1991) placed artificial seagrass units (ASUs)
inside, on the edge of, and outside seagrass beds.
Significant differences in copepod recruitment to
the ASUs were found as a result of border type,
and the importance of studying landscape features
of the seagrass in conjunction with fauna was emphasized (Bell and Hicks, 1991). Kurdziel and Bell
(1992) also used ASUs to demonstrate that meiofaunal copepods could utilize small patches as corridors
for dispersal within a landscape. In both these studies
the utilization of ASUs, which allow experimenters
to control blade density, blade length, and position
of patches, has been quite useful for examining responses of small organisms to landscape features.
Where patches of seagrass are large and continuous,
however, construction of ASUs becomes logistically
problematic, thereby rendering them relatively less
attractive experimentally.
2. Patchiness and Macrofauna
Studies of macrofauna and seagrass landscapes have
provided some of the best information on landscape